PETROLOGY OF SEDIMENTARY ROCKS

1. PETROLOGY OF SEDIMENTARY ROCKS Dr. Sugeng S Surjono Lab. Sedimentografi Jurusan Teknik Geologi, Fakultas Teknik Universitas Gadjah Mada

3. INTRODUCTION Three Rock Types: Igneous, Sedimentary, Metamorphic rocks Sedimentary rocks : rocks formed at the surface of the earth under low-temperature and low-pressure, result from the accumulation and solidification of sediments, material transported in water, air or ice (Raymond, 1995). Origin of sedimentary rocks: - Formation of source rocks/sediment source : intrusion, metamorphism, volcanism, tectonic uplift - Weathering : physical and chemical breakdown of source rocks - Erosion and Transportation agent of transportation : water, wind, ice - Deposition material is deposited within depositional basins - Diagenesis sediment is covered by successive layer of younger sediment; increased temperature and pressure leading to consolidation and lithification of the sediment into sedimentary rocks

4. INTRODUCTION • Sedimentary rocks are characterized by : • Presence of layers • Presence of transported grains • Sedimentary structures • Fossils Types of Sedimentary Rocks: (Tucker, 1991) Siliciclastic (fragmental) : - Conglomerates & breccias - Sandstones - Mudrocks Biogenic, biochemical and organic : - limestones & dolomites - cherts - phosphates - coal - oil shale Chemical : - evaporites - ironstones Volcaniclastic : (e.g.) ignimbrites, tuffs, hyaloclastites

6. Sedimentary rocks • Clastic (siliciclastic) rocks (80-85% of the stratigraphic record) • Carbonate sediments and rocks (10-15% of the stratigraphic record) • Volcaniclastic sediments and rocks • Others (< 5% of the stratigraphic record) : • Organic (carbonaceous) sediments and rocks • Evaporites

7. SILICICLASTIC SEDIMENTARY ROCKS Clast (from the Greek klastos, meaning ‘broken’) is the technical term for broken fragment within sedimentary rocks. It is also called as terrigenous grains Because most terrigenous grains are composed in part of silica, they are often referred to as siliciclastic grains. Siliciclastic sedimentary rocks are composed by clasts that originated from transportation and deposition of pre-existing rocks within depositional environments. Mechanism involved in the transportation include the wind, glaciers, river currents, waves, tidal currents, debris flow and turbidity currents (Tucker, 1991). Two important features of siliciclastic sediments related to depositional processes and diagenesis are sedimentary textures and structures. • DESCRIPTION OF SEDIMENTARY ROCKS (Prothero & Schwab, 2005): • Color • Sedimentary textures • Sedimentary structures • Composition • Fossil contents • Geometry of sedimentary rocks

8. Basic components of siliciclastic sedimentary rock are : -clasts or fragments -matrix -cements Color • Color usually reflects some aspect of the rock’s composition (bulk color can reflect the color of major mineralogical components) • Color of rock controlled by color of clast, matrix and cement • Color is not treated as an independent property, however, but as an aspect of sedimentary rock composition

9. Sedimentary textures Textures refers to the size, morphology, and arrangement (fabric) of siliciclastic grains that make up a sedimentary rock. Grain size • Grain or siliciclastic particles range in size from clay to boulder • The grade scale most widely used by sedimentologist is the Udden-Wenthworth scale • The Udden-Wentworth grain-size scale is based on factors of two:  = -log2 d ; where d is grain size in mm • - It extends from <1/256 mm (0.0039) to >256 mm and is divided into four major size categories (clay, silt, sand, and gravel) that can be further subdivided

10. Udden-Wenthworth grain-size scale for sediments and the equivalent phi scale Mud

11. Grain size parameters : mean, sorting, skewness, kurtosis • Grain-size (particle-size, granulometric) analysis • The old-fashioned way: direct measurement (gravel) and sieve/pipette analysis (sand and mud) • The modern technology: laser particle sizing (sand and mud)

12. Grain sizeparameters Graphical method Graphic Mean Standard deviation Skewness Kurtosis (Mz) = (σ1) = + (SK1) = + (KG) =

13. Grain size parameters Moment measures (mathematic method) • First moment: mean (cf. median, mode) • Premier measure of the grain size x ø = • Second moment: variance (cf. standard deviation) • Measure of the degree of sorting σø = ( = standard deviation) • Third moment : Skewness • - Measure of the symmetry of the grain-size distribution Skø = • Fourth moment : Kurtosis • - Measure of the sharpness or peakedness of a grain-size frequency curve Kø =

15. SK1 class : +1,0 - +0,3 very fine-skewed +0,3 - +0,1 fine-skewed +0,1 - -0,1 near-symmetrical -0,1 - -0,3 coarse-skewed -0,3 - -1,0 very coarse-skewed KG class: <0,67 very platykutic 0,67 – 0,90 platykurtic 0,90 – 1,11 mesokurtic 1,11 – 1,50 leptokurtic 1,50 – 3,00 very leptokurtic >3,00 extremely leptokurtic

20. Grain Morphology Three aspects of grain morphology are the shape, sphericity and roundness. • The shape or form of grain is measured by various ratios of the long, intermediate and short axes. • Sphericity is a measure of how closely the grain shape approaches that of a sphere. • Roundness is concerned with the curvature of the corners of a grain and six classes from very angular to well rounded.

21. Grain shape classification Roundness and Sphericity

23. Grain Fabric Fabric for grain in sedimentary rock refers to their orientation and packing and to the nature of contacts between them. • Grain Packing is a function of the size and shape of grains and postdepositional physical and chemical processes that bring about compaction of sediment. • Grain orientation is mainly a function of the physical processes and condition operating at the time of deposition

25. Sedimentary structures • Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally focused on the bedform-scale : Microforms (e.g., ripples) ;Mesoforms (e.g., dunes); Macroforms (e.g., bars) The majority of structures form by physical processes, before, during and after sedimentation. Other result from organic and chemical processes

27. Laminae and beds are the basic sedimentary units that produce stratification; the transition between the two is arbitrarily set at 10 mm • Normal grading is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow velocity), as opposed to reverse grading • Fining-upward successions and coarsening-upward successions are the products of vertically stacked individual beds

28. Cross stratification • Cross lamination (small-scale cross stratification) is produced by ripples • Cross bedding (large-scale cross stratification) is produced by dunes • Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output) • Straight-crested bedforms lead to planar cross stratification; sinuous or linguoid bedforms produce trough cross stratification

30. Low angle planar cross-bedding, Kali Ngalang-Gunung Kidul, YK

32. Cross stratification • The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘climbing ripple cross lamination’ • Antidunes form cross strata that dip upstream, but these are not commonly preserved • A single unit of cross-stratified material is known as a set; a succession of sets forms a co-set Planar stratification • Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditions • Planar stratification can easily be confused with planar cross stratification, depending on the orientation of a section (strike sections!)

34. Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and by laminae dipping in two directions • Hummocky cross stratification (HCS) forms during storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swales • Heterolithic stratification is characterized by alternating sand and mud laminae or beds • Flaser bedding is dominated by sand with isolated, thin mud drapes • Lenticular bedding is mud-dominated with isolated ripples

36. Gravity-flow deposits • Debris-flow deposits are typically poorly sorted, matrix-supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal direction • Turbidites, the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedforms

40. Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream • Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial • Scour marks (caused by erosive turbulence) • Tool marks (caused by imprints of objects) • Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts)

42. Trace fossils (ichnofossils) are the tracks, trails or burrows left behind in sediments by organisms (e.g., feeding traces, locomotion traces, escape burrows) • Disturbance of sediments by organisms is known as bioturbation, which can lead to the total destruction of primary sedimentary structures • Since numerous trace fossils are connected to specific depositional environments, they can be very useful in sedimentologic interpretations • Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include: • Slump structures (on slopes) • Dewatering structures (upward escape of water, commonly due to loading) • Load structures (density contrasts between sand and underlying wet mud; can in extreme cases cause mud diapirs)

47. Clastic (siliciclastic) rocks • Sandstones (20-25% of the stratigraphic record) can be subdivided according to the Pettijohn classification, based on texture and composition (relative proportions of quartz, feldspar, and lithic fragments) • Quartz arenite: quartz-dominated • Arkosic arenite: feldspar-dominated • Lithic arenite: dominance of lithic fragments • Wacke: significantly matrix-supported (>15% mud) • Quartz wacke • Greywacke (feldspathic or lithic wacke)

48. Classification of Sandstone (Pettijohn, 1975)

49. Clastic (siliciclastic) rocks • Mudstones (60% of the stratigraphic record) are also known as mudrocks or shales and commonly exhibit a distinct fissility • Claystone • Siltstone • Conglomerates are consolidated gravels; breccias are conglomerates with dominantly angular clasts • Clast-supported conglomerates • Matrix-supported conglomerates

50. Halang Formation, Panujah, Slawi, Central Jawa